STEAM GENERATORS.pptx

NAME : MALIK RAFIYAREYAZ
CLASS: BE 5TH SEM
BRANCH: MECHANICAL
ROLLNO: GCET-303-19 (191105040)
SUBJECT: APPLIED THERMODYNAMICS

CONTENTS
 INTRODUCTION
 CLASSFICATION OF BOILERS
 COMPARISION BETWEEN ‘FIRE-TUBE’ AND ‘WATER-TUBE’ BOILERS
 SELECTION OF A BOILER
 ESSENTIALS OF A GOOD STEAM BOILER
 BOILER TERMS
 FIRE TUBE BOILERS
 WATER TUBE BOILERS
 BOILER MOUNTINGS
 BOILER ACCESSORIES
 DRAUGHT
 PERFORMANCE OF STEAM GENERATORS

INTRODUCTION
A steam generator, popularly known as a boiler,
is a closed vessel made of high-quality steel in
which steam is generated from water by the
application of heat. The water receives heat
from the hot gases formed by burning fuel
through the heating surface of the boiler. Steam
is mainly required for power generation, process
heating, and space heating purposes.

DIFFERENCE BETWEEN FIRE TUBE AND WATERTUBE BOILERS
FIRE TUBE BOILER WATER TUBE BOILER
Hot flue gases flow inside the tube and the water outsides the
tubes.
Water flows inside the turbine and hot flue gases outside the tube.
These boilers are generally internally fired. These boilers are generally extrernally fired.
The boiler pressure limited to 20 bar. The boiler pressure is limited to up to 100 bar.
The fire-tube boiler has a lower rate of steam production. A higher rate of steam production.
Not suitable for larger power plants. Suitable for larger power plants.
Involves lesser risk of explosion due to low pressure. The risk of the explosion is higher due to high boiler pressure.

DIFFERENCE BETWEEN FIRE TUBE AND WATERTUBE BOILERS
Difficult in transportation. Simple in transportation.
They require less skill to operate, as compare to the
water tube boiler.
They required a skilled operator.
They are difficult to repair and cleaning as they are
internally fired.
They are easy to repair and clean as they are externally
fired.
They required a large shell diameter. Because the fire
tube situated inside the shell.
They required a small shell diameter.
The efficiency of the fire tube boiler is less as
compared to the water tube boiler.
The efficiency of the water tube boiler is more.
The maintenance of this boiler is costly. It requires
regular inspection.
They are easy to maintain as they are externally fired.
Ex: Cornish Boiler, Lancashire Boiler. Ex: Babcock and Wilcox Boiler.

DIFFERENCE BETWEEN FIRE TUBE AND WATERTUBE
BOILERS

SELECTION OF A BOILER
 High Pressure Vessel Mass
 High Water Volume
 High Efficiency
 Quick Startup Times
 Low Footprint
 Low Heat Flux and High Heating Surface
Area
 Ease of Clean Out
 Effective Turndown
 Ease of Maintenance
 Large Steam Disengagement Area

ESSENTIALS OF A GOOD BOILER
The essentials of a good boiler :-
 The boiler should be reliable and produce the required amount of steam per hour.
 The maintenance of the boiler should be easy and cheap.
 It should be efficient. It should have minimum fuel consumption and less initial
cost.
 It should start quickly.
 It should be light in weight, occupy less space and it should be able to meet the
variable demand of steam.
 The joints should be leak proof. The joints should be accessible for inspection and
these should be away from flame.
 The mud and other deposits should not collect on heated surface since this would
affect the heat transfer rates and consequently the steam generation rates.
 The refractory material used should be minimum.
 The design of boiler allow maximum fluid velocity in order to increase the heat
transfer rates between water and flue gases. It should have minimum friction
losses.
 It should comply with safety regulations according to Indian Boiler Act 1923.

BOILER TERMS
 Shell:- Shell is the main container that contains water for steam usually made
up of Steel plates bent into a cylindrical shape. The plates may be welded or
reverted together.
 Furnace:- Furnace is a firebox and is the space above the grate and below
boiler shell suitable area of furnace improves the efficiency of combustion.
 Grate:- It is the platform upon which the fuel burnt. Generally, the grate is
made up of cast iron bars evenly.
 Water flow path:- It is the path followed by the water in the boiler.
 Gas flow path:- It is the path followed by the flue gases of combustion. They
may be water tubes or fire tube boilers. The gas flow path should be so
arranged that it transferred maximum heat to the water to produce steam.
This much affects the efficiency of the boiler.
 Steam path:- Generally steam path is at the top of the shell to avoid water
particles being carried with the steam. To do so, steam separators are used in
case of large capacity and high-pressure boilers.

BOILER TERMS
 Fitting ( Mountings):- The valves and gauge which are necessary for the safety of the
boiler referred to as mountings. Without mountings, a boiler cannot function safely.
Example:- Water level indicator, Pressure gauge, Fusible plug, Safety valve, Blow off
cock, etc.
 Accessories:- The equipment which improves the efficiency of the boiler is known as
accessories.
Example:-
 Economizer and air preheater are used to extract maximum heat from the flue gases.
 Super heater used to raise the temperature of steam above the saturation
temperature.
 Blowing off:- The process of removing the impurities floating on the water surface
level is termed as blowing off. Some special device is used to blowing off impurities.
 Settings:- Settings are the walls of the Furnace and combustion chamber. These are
made up of fire bricks.
They form the passage through which the gases pass.
 Water space and steam:- The volume of the shell, which occupies the water is known
as water space. The level at which the water stands in the boiler shell is known as the
water level.
The remaining volume of Shell that not occupied by water is called steam space

FIRE TUBE BOILERS
These are one of the most fundamental and old
designed boilers. These boilers are very famous in the
18th century, and especially applicable for train
engines. In this kind of boiler, combustion of heat, as
well as gases, flows through the pipe enclosed by
water. These boilers may be high-pressure or low-
pressure boilers. The measurement of these boilers
can be done always with their external diameter.
Generally, these boilers are intended for pressures up
toward a highest of 250 psi & around 750 horsepower
These are one of the most fundamental and old
designed boilers. These boilers are very famous in the
18th century, and especially applicable for train
engines. In this kind of boiler, combustion of heat, as
well as gases, flows through the pipe enclosed by
water. These boilers may be high-pressure or low-
pressure boilers. The measurement of these boilers
can be done always with their external diameter.
Generally, these boilers are intended for pressures up
toward a highest of 250 psi & around 750 horsepower

FIRE TUBE BOILERS
Construction and Working Principle of Fire Tube Boiler
The construction of a water tube boiler can be equipped with
cylindrical shell, vertical, firebox in the base, space for
water in the center segment, and space for steam in the
higher segment. A fire-grate is located at the fire-box base as
well as coal is fired-up on the firebox. For burnt coal, an ash
pit is placed at the base of the grate for gathering the ash
from the burnt coal, and sometimes it can be detached.
One or many cross-tubes are flanged to the space of water
that is placed within the box to raise the outside area of
heating for improving the water flow. A small chimney is
associated with the pinnacle of the firebox for releasing the
waste outlets at some larger height. The cleaning of the
boiler can be done by the hand holes as well as manholes of
the tubes & shell of the boiler.

Construction and Working Principle of Fire Tube Boiler
 These boilers include a water level indicator; a
pressure gauge, steam stop tap security tap, & a
manhole like mountings for providing security as well as
simplicity of working. The Fuel burns on the grate of
the firebox in the boiler and the resultant hot-flue
gases are permitted to flow in the region of cross
tubes.
 The water nearby the cylindrical type firebox also gets
heat through radiation as well as convection, therefore
steam will be generated. The flow of water in the
boiler will be based on the difference of density within
water that is formed with the difference of
temperature.
FIRE TUBE BOILERS

FIRE TUBE BOILERS
Types of Fire Tube
Boiler
 The different types of this boiler
include the following.
1) Cornish Fire Tube Boiler
The first Cornish boiler was approved
by an engineer namely “Trevithick a
Cornish”. This kind of boiler includes
a plane cylindrical shell, as well as a
tiny flue pipe holding the heating
system, flows through it.
2) Lancashire Fire Tube Boiler
The construction of the Lancashire
boiler is related to Cornish boiler but
as a substitute of one flue pipe, two
flue pipes are utilized.

FIRE TUBE BOILERS
3) Locomotive Fire Tube Boiler
The locomotive boiler is an inactive boiler
which is used in train engines. This type of
boiler is capable while producing steam as
well as it is solid. The design of the
locomotive boiler is horizontal multi
tubular. The main advantage of using this
type of boiler is low cost for construction,
installation and steam capacity is high.
4) Vertical Fire Tube Boiler
The vertical boiler is a simple boiler, and it
comprises of a cylindrical shell enclosed a
larger section of water and rest of section
will be occupied with steam. It contains
cross tubes and a furnace at the bottom of
the boiler. The combustion gases after
heating the water are allowed to escape
into the atmosphere.

FIRE TUBE BOILERS
5) Cochran Fire Tube Boiler
The Cochran boiler is a vertical type
multi-tubular boiler, and it includes
several horizontal fire tubes. The heating
system is one part of the construction and
it is perfect.
6) Scotch Marine Fire Tube Boiler
The Scotch marine boiler is very popular
boilers used for high vapor capacities at
high forces. This type of boiler includes a
huge number of tiny diameter tubes for
providing the benefit of the high-heating
region of the surface. The boilers are
fired up internally and leave from the
boiler using a chimney to the
environment.

WATER TUBE BOILERS
A water tube boiler can be
defined as a Steam boiler in
which the flow of water in
the tubes, as well as hot
gases, enclose the tubes. Not
like fire tube boilers, this
boiler attains high-pressures,
as well as high-steam
capabilities, can be achieved.
This is because of condensed
tangential pressure on tubes
which is known as hoop
stress.

Construction and Working Principle of Fire Tube Boiler:
The working principle of water tube boiler is thermal siphoning
(circulation of natural water). Basically, this type of boiler
includes two drums namely steam, lower or mud drum.
Two drums are associated via two tubes such as down comer and
riser. At first, the water is supplied into the steam type drum with
the help of a water pump. Whenever the fuel is burned, then hot
gases will be generated that are permitted to supply in the shell
part of the boiler. The hot gases which are produced by the fuel
will replace heat by the water; the water gets changed into
steam. Because, the water temperature increases, the
concentration will increase automatically.
Clearly, the concentration of steam will be lesser than the water.
Thus in the steam drum, the water, as well as steam, gets divided
obviously due to variation in concentration. Here the traveling of
steam will be upward because of low concentration as well as
water will travel downward because of high concentration.
WATER TUBE BOILERS

WATER TUBE BOILERS
Construction and Working Principle of Fire Tube Boiler:
The flow of hot water at the base of the steam drum will be supplied
into mud drum via downcomer tube as well as to heat the water in the
mud type drum. Whenever cooler supply water is initiated into steam
type drum due to the high concentration of cool water, and it moves
down in the downcomer type tube near mud drum.
When transferring hot water from the mud type drum to steam type
drum using riser tubes then it consequences in normal water circulation
in the water tube type boiler. As additional steam is generated in the
boiler then the force in the drum enhances & the water supply into the
drum reduces which tend for decreasing the flow of steam. In the same
way, when the production rate of steam reduces the force in the water-
tube boiler reduces, & the water supply into the boiler enhances which
tends to enhance the production rate. Like this, the water tube boiler
controls the production of steam.

WATER TUBE BOILERS
Types of Water Tube Boilers
The types of water tube boilers include the
following.
1) Simple Vertical Boiler
This is one type of water tube boiler. In
this type of boiler, the axis of direction
is perpendicular with respect to the
position. The major components of this
boiler include ash pit, grate, feed check
tap, fire hole, firebox, cross box, hand
hole, fusible plug, water gauge,
cylindrical shell, steam space, manhole,
pressure gauge, steam stop tap, safety
tap, chimney.
In this type of boiler, using the fire hoke
fuel is added into the grate which
destroys by fire to generate the warm
gases. Ash pit is for collecting the ash
which is converted from the fuel. Hot
gases increase high and supply their heat
toward the water within the cross box,
then moves out using the chimney.

2) Stirling Boiler
The Stirling boiler is one type of water tube
boiler, used for generating steam (50,000 kg
steam/hour and 60 kgf /cm2 pressure) in the
large area of the stationary plant. This type of
boiler consists of 3 steam drums as well as 2 mud
drums. The steam drums are located on the top
section of the boiler whereas mud drums are
located on the base of the arrangement. The
steam drums and mud drums are connected
through bent tube banks.
When the tubes are turned then the mechanical
pressures due to pipes extension throughout
heating cannot influence the system. The two
drums as well as tubes are designed of steel
which will support the total system.
The arrangement of the Stirling boiler is enclosed
with brickwork. Here, the arrangement of bricks
will avoid the heat dissipation in the surroundings
WATER TUBE BOILERS

3) Babcock and Wilcox Boilers
This is a horizontal straight water tube boiler; it has a
steam drum which is made of steel. The two ends of
the drum are associated with a series of two end
headers with short riser pipes. These are disposed at
15o0 angle to the horizontal axis of the steam drum.
The disposed of the arrangement of tubes assists in
the supply of water, and the water level in the drum
is indicated with a water level indicator.
The fire door in the water boiler will be located at
the bottom and the fuel will be supplied through this
door and burns in a grate. The burning fuel will
generate the hot gases that are forced in the grate to
supply upward with baffle plates. At the bottom of
the water boiler, mud collector is located for
removing the mud particles using blow down a cock.
Therefore, a nonstop water circulation from the
steam drum toward the water tubes is maintained by
convective currents because of the difference in
concentration and it is called as normal circulation
WATER TUBE BOILERS

BOILER MOUNTINGS
 Regular accidents and boiler
break down can happen due to
improper maintenance and
cleaning. Without boiler
mounting one can damage boiler
and its surrounding with one
mistake followed by a series of
events. Without high lift safety
valve the shell could explode
resulting into great loss of life
and machinery; similarly a shell
could collapse under vacuum if
air vent is not present during
cooling down. During cargo
operation there is a increased
demand for steam and so the
mountings help operate the
boiler with more and ease and
safety. Boiler mountings include:

1 ) Safety Valves
Safety valves are fitted on every boiler to avoid
over pressurizing. Normally three safety valves
are fitted on the boiler with one on the super
heater and rest two on steam drum. In no
condition these valves be less than two in
number and must lift at a pressure 3% above
boiler working pressure irrespective of boiler
types.
A steam valve is made up of cast iron body with
two independent valves fitted on the valve seat.
These valves are connected to a lever by means
of a pivot held tight to its position by spring.
The spring force keep the valve sit shut on the
valve seat under normal condition. When the
upward pressure exceeds the downward spring
force; the valve is lifted and excess steam is
released to the atmosphere.
2 ) Vent Valve
Vent valve is installed on the boiler shell to vent
air from steam drum during starting of boiler.
These vent valves also comes handy during
boiler shut down as it let fresh air to enter the
boiler drum avoiding its collapse under pressure.
A vent valve can also be used to release / dump
moist steam at start.
BOILER MOUNTINGS

3 ) Fusible Plug
A fusible plug is the threaded gunmetal cylinder with conical plug and tappet hole
drilled into it. This This hole is then filled with an alloy of low melting point such as tin.
The plug can be of either fire actuated or steam actuated type fitted over the
combustion chamber.
Under normal condition one side of the plug is exposed to extreme temperature while
the other submerged under water; keeping it cool. This low melting point alloy can not
melt away till submerged and so remain intact even under extreme condition. In event
of water level reach below a safe limit and plug tip is exposed to steam; the tin alloy
will melt exposing combustion chamber with steam. As steam is not effective coolant
and convection medium the tin alloy can’t transfer heat to the steam leading it to melt
away. This sudden injection of steam into the furnace will stop the combustion
protecting boiler from any damage.
4) Steam Stop valve
A steam stop valve is connected to the boiler to stop and regulate steam flow from
boiler to the distribution lines. Main steam stop valve on boiler is kept shut to avoid
back-flow of steam to the boiler. The flange of the steam stop valve is bolted on top of
the steam drum. Valve main body is made of cast iron while the valve seat is made from
gun metal. The spindle on one end is connected to the valve while the other end to the
handle wheel passing through ( yoke / Gland nut ) and gland packing. The valve is
operated by rotating the hand wheel. Rotating hand wheel in turn rotate the spindle
which move the valve up allowing path for steam to flow.
BOILER MOUNTINGS

5 ) Pressure Gauge
Pressure gauge are fitted to the steam drum and super heater to indicate steam pressure
inside. These gauge are fitted on the front top of the boiler shell and represent pressure
in bar. A bourdon tube of closed cross section is attached to the steam space on one end
through siphon tube. The tube itself contain water to avoid steam to enter into the
pressure gauge. The pointer is connected to the threaded gear attached to the spindle.
When pressure is applied to the bourdon tube it becomes circular turning the spindle.
This cause the pointer to move along with the gear; representing the boiler pressure.
6 ) Water Level Indicator
A pair of water level indicator is installed directly to the boiler shell with an additional
remote reading gauge installed at convenient position. They are installed directly on the
front end of all boiler types; showing water level in boiler drum. It consist of a glass tube
with three independent cock ( Steam cock, water cock and drain cock ).
Steam and water cock separates the glass tube with boiler steam and water respectively.
Drain cock on other hand used to drain water from glass tube. A metal ball is provided on
the water side of the gauge glass to avoid subsequent accident and water loss; by water
flashing off steam in event of glass rapture / failure.
Under normal condition both steam and water cock is open allowing water and steam
pressure to balance. In event of incorrect reading we need to blow through; by closing
the water cock and opening drain cock. A strong blow will indicate the steam cock is
clear; now repeat the process with steam cock closed and water cock opened. Strong
blow of steam with hissing sound indicate the water cock is clear. Now close the drain
cock and let water fill in; slowly open the steam cock equalizing the pressure.
BOILER MOUNTINGS

BOILER ACCESSORIES
The boiler accessories are the devices, which form
an integral part of a boiler but are not mounted on
it. They include superheater, economiser, feed pump
etc. It may be noted the accessories help in
controlling and running the boiler efficiently

1) Economiser
The combustion gases coming out of the boiler
contain a large quantity of heat. Therefore the
maximum amount of heat from the gases should
be recovered before it escapes to the chimney.
In the economiser, heating the feed water does
the recovery of heat in the flue gases. The
economiser is placed in the path of the gases.
They improve the overall efficiency of the boiler
by reducing fuel consumption.
2) Air Pre-heater
The air preheater is an accessory that recovers
the heat in the exhaust gas by heating the air
supplied to the furnace of the boiler. Supplying
preheated air into the furnace produces a high
furnace temperature and accelerates the
combustion of the fuel. Thus the thermal
efficiency of the plant will be increased.
BOILER ACCESSORIES

3) Superheater
The superheater is used in boilers to increase the
temperature of the steam above the saturation
temperature
The dry saturated steam generated in the boiler
is passed through a set of tubes placed in the
path of the flue gases, in which it will be heated
further by the hot gas to increase its
temperature about the saturation temperature
4)Feed Pump
A feed pump is a boiler accessory required to
force the feed water at high pressure into the
boiler. Commonly used pumps are,
 Reciprocating pumps
 Rotary pumps
The reciprocating pumps are driven directly by
coupling them to the steam engine. The rotary
pumps are driven by the steam turbines or by
electric motors.
BOILER ACCESSORIES

DRAUGHT
Boiler draught is defined as the difference between absolute gas pressure at any
point in a flow passage and the ambient (same elevation) atmospheric pressure.
Draught is achieved a small pressure difference which causes the flow of air or
gas to take place. It is measured in millimeter (mm) or water.
The draught is one of the most essential systems of the thermal power plant
which support the required quantity of air for combustion and removes the burnt
products from the system. To move the air through the fuel bed and to produce a
flow of hot gases through the boiler economiser, preheater and chimney require
a difference of pressure.
This difference of pressure to maintaining the constant flow of air and
discharging the gases through the chimney to the atmosphere is known as
draught. Draught can be achieved by the use of chimney, fan, steam or air jet or
a combination of these.
When the draught is produced with the help of chimney only, it is known
as Natural Draught and when the draught is produced by any other means except
chimney it is known as Artificial Draught

Purpose of Boiler Draught:
 To provide an adequate supply of air for fuel combustion.
 For throw out the exhaust gases of combustion from
the combustion chamber.
 To discharge these gases to the atmosphere through the
chimney.
DRAUGHT

Measurement of Draught
The amount of draught produce depends upon:
 The nature and depth of fuel at the furnace.
 Design of combustion chamber or firebox.
 The rate of combustion required.
 Resistance is allowed in the system due to baffles, tubes, superheaters,
economizers, air pre-heaters etc.
DRAUGHT

DRAUGHT
Classification of Boiler Draught

DRAUGHT
Types of Boiler Draught
In general, the draughts may be classified into the following two types,
 Natural Draught
 Artificial Draught
Natural Draught
Natural draught system employs a tall chimney as shown in the figure. The chimney is a vertical tubular masonry
structure or reinforced concrete. It is formed for enclosing a column of flue gases to produce the draught.
It removes the gases high enough to prevent air pollution. The draught is produced by this tall chimney due to the
temperature difference of hot gases in the chimney and cold external air outside the chimney.
Artificial or Mechanical Draught
It has been seen that the draught produced by the chimney is affected by the atmospheric conditions. It has no
flexibility, poor efficiency and tall chimney are required. In most of the modern power plants, the draught applied
must be freedom of atmospheric condition, and It should have more flexibility (control) to bear the fluctuation
loads on the plant.
Today’s steam power plants requiring 20 thousand tons of steam per hour would be impossible to run without the
aid of draft fans. A chimney of a reasonable height would be incapable of improving enough draft to eliminate the
huge volume of air and gases ( 400 x 103 m 3 to 800 x 10 3 m 3 per minutes). The further advantages of fans are to
reduce the height of the chimney needed.

DRAUGHT
Determination of the Height of
Chimney for a Given Value of
Natural Draught
For determination of the height of
chimney required to produce certain
amount of natural draught, we have to
determine the density of flue gases inside
the chimney, ρg and that of cold air
outside the chimney ρa.
A shown in fig 29.1, if H is the height of
chimney and Pa is the atmospheric
pressure at the top level of the chimney
then
The absolute total pressure at grate level
inside the chimney is given as

DRAUGHT
Determination of the Height of Chimney for a Given Value of Natural Draught
And total pressure at grate level outside the chimney
Then draught produced, given by its definition will be equal to
Where, ρa and ρg are in kg/m3
g = acceleration due to gravity = 9.81 m/sec2
H = Height of chimney in m
Calculation of ρa and ρg
We know the combustion chemistry of carbon and hydrogen
N/m2 (Pa)

DRAUGHT
From equation (i) & (ii) it is clear that volume of O2 supplied for combustion of a
unit quantity of carbon is equal to volume of CO2 produced at same pressure and
temperature and volume of O2 supplied for combustion of hydrogen is half of
volume of steam produced at same temperature and pressure and volume of
N2 supplied remains same at same temperature and pressure, as it does not take
part in combustion.
Now, the hydrogen content in the fuel is very less because of which only, the
volume of combustion product increases. So, it can be neglected and it is safely
assumed that volume of products of combustion will be equal to volume of air
supplied when both reduced to same temperature and pressure conditions
Let m = mass of air required per kg of fuel burnt
Let m = mass of air required per kg of fuel burnt
Let T = mean absolute temperature of chimney gases in K
T1= Absolute temperature of outside air in K

DRAUGHT
Where, P is in N/m2, T in oK and Ra In J/kg K
And volume of (m+1) kg of flue gases will also be same at temperature T1 i.e
But hot flue gases are at temperature T. So their volume at temperature T will be
Here pressure P is in N/m2, Ra in J/kg K and temperature in K
Putting the value of density of fresh air at temperature T1 and of hot flue gases at
temperature T2 in equation
(Considering atmospheric Pressure Pa, Constant)
Density of inside flue gases

DRAUGHT
Putting the values of
Atmospheric Pressure P = 101300 Pa (N/m2)
Ra = Characteristic gas constant for air = 287 J/ kg K
and g = Acceleration due to gravity = 9.81 m/sec2
Draught Produced
By this formula the theoretical height of chimney required may be determined for producing a
certain amount of natural draught, if we know the absolute temperature of flue gases inside the
chimney and of air outside the chimney and mass flow rate of air per kg of fuel burnt.
Actual Draught produced by chimney is generally less than 12 mm of water.
From equation 29.2, it may be seen that the draught produced by the chimney becomes more
effective with increase in height of chimney and temperature of flue gases

DRAUGHT
Determination of Diameter of Chimney
Let us find out the draught produced in terms of height of flue gases i.e. h meters.
So, it will be expressed as pd = ρg. g. h
And From Equation
Putting the values of ρg from sub eqn (ii)
Now assuming that draught losses are Nil and all the draught contribute to imparting
velocity to flue gases, velocity of flue gases can be given by:
If we take into account the frictional pressure losses in chimney and let it be hf, then

DRAUGHT
Determination of Diameter of Chimney
Where k is a constant the value of which is given as
k= 0.825 for brick chimney, 1.1 for steel chimney
Also, the mass of flue gases flowing through any section of chimney will be given by
In this way, the height and diameter of chimney to produce a
given natural draught can be estimated theoretically

DRAUGHT
Efficiency of Chimney
From equation 29.3, it is clear that for a given height of chimney draught depends on
temperature of hot gases leaving the chimney. Natural Draught is directly proportional to outlet
temperature of flue gases. It increases with the increase in outlet temperature. But high
temperature of hot gases leaving chimney means the loss of heat energy and hence thermal
efficiency of boiler. Thus natural draught is created at the cost of thermal efficiency.
However in artificial draught system, the flue gases can be made to leave the chimney at a
reduced temperature, thus increasing the thermal efficiency or we can say that more heat of
flue gases can be harnessed and given to steam. Thus the efficiency of chimney can be
calculated on this basis:
Let T = Temperature of flue gases in natural draught
T2 = Temperature of flue gases in artificial draught
Cp = mean sp heat of flue gases in J/kg K
Then extra heat carried away by flue gases due to higher temperature required for producing
natural draught Cp (T , T2) for one kg of flue gases
Draught Produced in terms of height of column of exhaust gases

DRAUGHT
Efficiency of Chimney
It can give maximum energy to one kg of flue gases = m. g. h
Thus efficiency of chimney can be calculated as
It is to be noted that even for a very tall chimney, the efficiency, will be less than 1%. So it is a
very inefficient mean to create draught. Due to this reason for and for demand of energy
efficiency, natural drought is not used in most of the commercial boilers.

PERFORMANCE OF STEAMGENERATORS
1) Equivalent evaporation
It is defined as the weight of saturated water at 100C evaporated to dry and saturated steam at
100C by utilizing heat at the same rate as would have been used under the actual working
conditions.
If H = Total heat of steam at gHw1 = Total heat of feed water in kJ/kg
L = Latent heat of steam at atmospheric pressure = 2257 kJ/kg
Wa = Weight of steam produced per hour at given working pressure per kg of fuel
We = Equivalent evaporation in kg per kg of fuel
Then as per the above definition, We L = Wa (H , Hw1)
Given working pressure in kJ/kg
= Wa .F, where F= = Wa .F, where F

PERFORMANCE OF STEAMGENERATORS
2) Factor of evaporation
It is a quantity which when multiplied by the actual amount of steam generated at a given
pressure from water at given temperature, gives the equivalent evaporation.
Equivalent Evaporation = F x Actual Evaporation
Thus factor of evaporation may also be defined as ratio of Equivalent Evaporation to actual
Evaporation.
3) Boiler efficiency
It is the ratio of heat actually utilized in generation of steam in a given period to the heat
supplied by fuel in the same period or it is the ratio of heat utilized in generation of a given
quantity of steam to the heat supplied by fuel burnt to produce this steam.
Where C = calorific value of fuel in kJ/kg and Wf = weight of fuel per hour.
Boiler efficiency is always less than 1 because of some loss of heat through hot gases escaping
to atmosphere and also directly to atmosphere by conduction convection and radiation.

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